Restore GCC 10 test results, and add diagnostic for the one construct that's known not to work on GCC 10

Although I'd previously increased the minimum level of GCC support from 10.x to 11.x, in practice most things work just fine. The only known exception is a `forward` parameter of a non-wildcard type on a type-scope (aka member) function.

So this commit:
- restores the `test-results/gcc-10` directory (while still keeping the `gcc-13` one too)
- adds a check for the known case that doesn't work in GCC 10

I don't intend to do a lot of work to support downlevel compilers, but if it's just one rarely-used item then it's worth trying to maintain downlevel compatibility. If we encounter other weaknesses with GCC 10, I'll likely just re-raise the minimum GCC version back to 11.

Author: hsutter

include/cpp2util.h

@@ -1755,8 +1755,10 @@ using cpp2::cpp2_new;
 // See commit 5a0d77f8e297902c0b9712c5aafb6208cfa4c139.
 #if !defined(__clang__) && defined(__GNUC__) && __GNUC__ == 10
     #define CPP2_REQUIRES(...) /* empty */
+    #define CPP2_REQUIRES_MEMFN(...) static_assert(false, "GCC 11 or higher is required to support type-scope functions that have a 'forward' parameter of non-wildcard type, such as 'func: (this, forward s: std::string)' - if you must use GCC 10, use 'forward s: _' instead")
 #else
     #define CPP2_REQUIRES(...) requires (__VA_ARGS__)
+    #define CPP2_REQUIRES_MEMFN(...) requires (__VA_ARGS__)
 #endif
 
 #endif

regression-tests/test-results/gcc-10/gcc-version.output

@@ -0,0 +1,5 @@
+g++-10 (Ubuntu 10.3.0-1ubuntu1~20.04) 10.3.0
+Copyright (C) 2020 Free Software Foundation, Inc.
+This is free software; see the source for copying conditions.  There is NO
+warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+

regression-tests/test-results/gcc-10/mixed-allcpp1-hello.cpp.execution

@@ -0,0 +1 @@
+Hello world

regression-tests/test-results/gcc-10/mixed-allcpp1-hello.cpp.output

@@ -0,0 +1,60 @@
+v as X< 0> = 0
+v as X< 1> = 1
+v as X< 2> = 2
+v as X< 3> = 3
+v as X< 4> = 4
+v as X< 5> = 5
+v as X< 6> = 6
+v as X< 7> = 7
+v as X< 8> = 8
+v as X< 9> = 9
+v as X<10> = 10
+v as X<11> = 11
+v as X<12> = 12
+v as X<13> = 13
+v as X<14> = 14
+v as X<15> = 15
+v as X<16> = 16
+v as X<17> = 17
+v as X<18> = 18
+v as X<19> = 19
+as_const(v) as X< 0> = 0
+as_const(v) as X< 1> = 1
+as_const(v) as X< 2> = 2
+as_const(v) as X< 3> = 3
+as_const(v) as X< 4> = 4
+as_const(v) as X< 5> = 5
+as_const(v) as X< 6> = 6
+as_const(v) as X< 7> = 7
+as_const(v) as X< 8> = 8
+as_const(v) as X< 9> = 9
+as_const(v) as X<10> = 10
+as_const(v) as X<11> = 11
+as_const(v) as X<12> = 12
+as_const(v) as X<13> = 13
+as_const(v) as X<14> = 14
+as_const(v) as X<15> = 15
+as_const(v) as X<16> = 16
+as_const(v) as X<17> = 17
+as_const(v) as X<18> = 18
+as_const(v) as X<19> = 19
+move(v) as X< 0> = 0
+move(v) as X< 1> = 1
+move(v) as X< 2> = 2
+move(v) as X< 3> = 3
+move(v) as X< 4> = 4
+move(v) as X< 5> = 5
+move(v) as X< 6> = 6
+move(v) as X< 7> = 7
+move(v) as X< 8> = 8
+move(v) as X< 9> = 9
+move(v) as X<10> = 10
+move(v) as X<11> = 11
+move(v) as X<12> = 12
+move(v) as X<13> = 13
+move(v) as X<14> = 14
+move(v) as X<15> = 15
+move(v) as X<16> = 16
+move(v) as X<17> = 17
+move(v) as X<18> = 18
+move(v) as X<19> = 19

regression-tests/test-results/gcc-10/mixed-as-for-variant-20-types.cpp.execution

@@ -0,0 +1 @@
+Bounds safety violation: out of bounds access attempt detected

regression-tests/test-results/gcc-10/mixed-as-for-variant-20-types.cpp.output

@@ -0,0 +1,5 @@
+1
+44
+45
+46
+5